1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by electric field effect transistors (FETs) such as thin-film transistors (TFTs) and to a method of fabricating the same. TFT stands for a semiconductor element including a semiconductor layer, a gate electrode, a source electrode and a drain electrode.
In this specification, an element substrate stands for a substrate, in general, in which semiconductor elements are formed.
In this specification, a display device stands for a device, in general, which produces a bright/dark display relying upon changes in the electric signals, and a device which produces a display by applying electric signals to the liquid crystals is called a liquid crystal display device.
2. Description of the Related Art
In recent years, attention has been given to a technology for constituting TFTs by using a thin semiconductor film (several nm to several hundred nm thick) formed on a substrate having an insulating surface. The TFT is widely applied to electronic devices such as ICs and semiconductor devices, and it has been desired to develop the TFT as a switching element particularly for the liquid crystal display devices.
Known liquid crystal display devices can be roughly divided into two types; i.e., those of the active matrix type and those of the passive matrix type. A liquid crystal display device of the active matrix type uses TFTs as switching elements and is capable of displaying a high quality. The liquid crystal display device of the active matrix is usually used for the notebook-type personal computers, but it is expected that it can also be used for household TVs and portable terminals.
Among the liquid crystal display devices of the active matrix type, the liquid crystal display device of the projection type is capable of producing a display of a large size by expanding the picture on a screen. Concerning the liquid crystal display device of the projection type, technology has recently been developed to realize the portable devices by decreasing the size of the optical system by designing the liquid crystal display panel in a small size. A decrease in the size of the optical system helps lower the cost of the optical system and, hence, makes it possible to cheaply provide a liquid crystal display device.
The liquid crystal display device of the active matrix type is generally reverse-drives the lines. Among the reverse-drives of the lines, the reverse-drive of the source lines is the one in which as shown in a schematic diagram of FIG. 21, the polarity of a signal voltage written into the pixel TFTs connected to the signal lines of m columns is differed for each of the neighboring signal lines. The polarities of the signal voltages written into the pixel TFTs connected to the signal lines are changed depending upon the frames of odd numbers (FIG. 21A) and the frames of even numbers (FIG. 21B). Upon alternatingly driving the liquid crystals by changing the polarities of the signal voltages written into the pixel TFTs, the liquid crystals are prevented from being printed. The reverse-drive of gate lines is executed by replacing the signal lines of FIG. 21 by the scanning lines.
On the interface of the oriented film, the liquid crystals are so oriented as to lift up an end thereof. In this specification, a direction from an end close to the interface of the oriented film of liquid crystal molecules toward an end lifted up from the oriented film, which is orthogonally projected onto the surface of the substrate, is referred to as xe2x80x9cpretilted directionxe2x80x9d. Further, an angle subtended by the interface of the oriented film and by the long axis of liquid crystals near the interface of the oriented film, is referred to as xe2x80x9cpretilted anglexe2x80x9d. The pretilted angle is imparted by either the rubbing or the switching of liquid crystals near the interface of the oriented film by applying an electric field to the liquid crystals.
In this specification, further, the defective orientation that stems from nearly the reversed pretilted direction of the adjacent liquid crystals on the interface of the oriented film is referred to as xe2x80x9cdisclinationxe2x80x9d. Further, though the pretilted direction of the liquid crystals is the same, there exists a region where the pretilted angle locally differs due to the electric field distribution and the irregular rubbing. The defective orientation of liquid crystals that develop when the orientation is not normal turns out to be locally bright like the leakage of light when the two pieces of polarizer plates are arranged on the liquid crystal panel. The orientation of liquid crystals in which the pretilted direction is the same but in which the pretitlted angle is locally different, is referred to as xe2x80x9cleakage of lightxe2x80x9d in this specification.
When the liquid crystal display device is driven by the active matrix system, the quality of display is spoiled by the leakage of light and disclination. That is, in the normally white mode, a light-shielding film is necessary for concealing the leakage of light and the disclination, and the numerical aperture drops.
In the liquid crystal display device in which fine pixels are formed such as the one of the projection type, the disclination and the leakage of light occur at a ratio which is no longer negligible relative to the pixels. Further, as the leakage of light and the disclination are not all concealed due to the deviation in the alignment of the light-shielding film, the leakage of light like bright line and the disclination are seen at the time of black display, and the contrast drops. That is, in the liquid crystal display device of the projection type, what is important is how to suppress the leakage of light and the disclination.
As compared to the smectic liquid crystals having a layered structure and a highly oriented order, the nematic liquid crystals tend to develop the disclination and the leakage of light due to an electric field established between a pixel electrode and another pixel electrode. In the orientation system using nematic liquid crystals, therefore, it is necessary to take a countermeasure to lower the disclination and the leakage of light.
How the leakage of light and the disclination occur will now be described with reference to FIG. 18 which is a sectional view schematically illustrating the pixel portion of the liquid crystal display device. Between the neighboring pixel electrodes in FIG. 18, it is now presumed that a first pixel electrode 101a has a potential of +5 V and a second pixel electrode 101b has a potential of xe2x88x925 V. Let it now be presumed that an opposing electrode 102 has a potential of 0 V. In a region where the equipotential lines 103 are in parallel with the surface of the pixel electrode, the liquid crystals of the positive type are so oriented that the long axes of the liquid crystal molecules 108 are perpendicular to the surface of the pixel electrode. The liquid crystals of the positive type stand for the liquid crystals having a positive dielectric anisotropy. At the end of the pixel electrode, however, the equipotntial lines are bent, and the liquid crystal molecules 106 are oriented aslant with respect to the surface of the pixel electrode, i.e., are defectively oriented. It is considered that how to lower the bending of equipotential lines at the end of the pixel electrode is important from the standpoint of lowering the defective orientation.
At an end of the pixel electrode, there exists a region 104 of leakage of light where the pretilted angle locally differs. Since the equipotential lines are bent at the end of the pixel electrode, the liquid crystal molecules 106 at the end of the pixel electrode cannot be so switched that the long axes thereof become perpendicular to the surface of the pixel electrode.
Further, there exists a region where the pretilted direction of the liquid crystals becomes opposite to the pretilted direction determined by the rubbing direction 107 due to the electric field established at an end of the pixel electrode. Then, the pretilted angle and the pretilted direction locally change sharply on the interface of the oriented film, whereby the orientation of the liquid crystals is greatly distorted and the disclination occurs in the region 105.
That is, the disclination and the leakage of light are caused as the equipotential lines that are in parallel with the surface of the pixel electrode are bent at an end of the pixel electrode. In the invention described below, a structural contrivance is made so as to suppress the bending of equipotential lines as much as possible at the end of the pixel electrode.
It is an assignment of the present invention to provide an element structure which is capable of preventing the leakage of current and the disclination in the liquid crystal display device of the active matrix type.
In this specification, the height of a dielectric stands for a distance between the surface of the pixel electrode with which the dielectric comes into contact and the uppermost end of the dielectric. In this specification, further, the cell gap stands for a distance between the surface that comes in contact with the opposing electrode and the surface that comes in contact with the main surface of the pixel electrode. The main surface of the pixel electrode stands for a flat surface that occupies not less than 30% and, preferably, not less than 50% of the pixel electrode. That is, the main surface of the pixel electrode stands for a flat surface that occupies a maximum area of the pixel electrode.
FIGS. 5A to 5C illustrate a principle of this invention. FIG. 5 is a sectional view of a pixel portion in the liquid crystal display device. Referring to FIG. 5A, a first pixel electrode 901a and a second pixel electrode 901b are provided on a flat surface. An opposing electrode 902 is provided facing the pixel electrodes. At the end of the pixel electrode, equipotential lines 903 are bent toward the pixel electrode causing the occurrence of disclination and leakage of light.
Referring to FIG. 5B, a dielectric 904 of a high dielectric constant is formed on the ends of the pixel electrodes. With the dielectric 904 of the high dielectric constant and a dielectric of a low dielectric constant, i.e., liquid crystals being connected in series at the ends of the pixel electrodes, a voltage is reluctantly applied to the dielectric of the high dielectric constant. With the dielectric 904 of the high dielectric constant being provided at the ends of the first pixel electrode 901a and of the second pixel electrode 901b, the voltage is reluctantly applied to the dielectric of the high dielectric constant. Accordingly, the equipotential lines are lifted on the dielectric of the high dielectric constant toward the opposing electrode 902. That is, upon providing the dielectric of the high dielectric constant on the ends of the pixel electrodes, there is produced an effect for suppressing the equipotential lines from bending at the ends of the pixel electrodes. The components of equipotential lines in parallel with the surface of the pixel electrode increase resulting in an increase in the electric field component in a direction perpendicular to the surfaces of the pixel electrodes.
Referring to FIG. 5C, when the height of the dielectric 904 is too great, the equipotential lines 903 swell conspicuously toward the opposing electrode 902, which is detrimental to orienting the liquid crystals. Namely, there exists an optimum value concerning the height of the dielectric.
It is considered that the region where the disclination and the leakage of light occur is the region where the equipotential lines are bending relative to the surfaces of the pixel electrodes. Therefore, the dielectric of the high dielectric constant should be formed in the region where the disclination and the leakage of light occur to suppress the bending of equipotential lines.
FIG. 2 is a model of simulation illustrating, in cross section, the pixel portion of the liquid crystal display device, wherein the device is simulated by providing a dielectric 304 having a relative dielectric constant of 30 on a first pixel electrode 303a and on a second pixel electrode 303b. The dielectric has a height (h) of 0.5 xcexcm and a width, in cross section, of 6.0 xcexcm. The dielectric 304 is formed being overlapped on the first pixel electrode and on the second pixel electrode over an equal width (L). The width (L) over which the dielectric 304 is overlapped on the first pixel electrode and on the second pixel electrode is 2.0 xcexcm. The potential of the first pixel electrode is +5 V, the potential of the second pixel electrode xe2x88x925 V, and the potential of the opposing electrode 301 is 0 V. A cell gap (d) is 4.5 xcexcm. The device is simulated by using physical values of ZLI4792 (manufactured by Merc Co.) at room temperature. The ZLI4792 exhibits a relative dielectric constant of 8.3 in the direction of long axis and a relative dielectric constant of 3.1 in the direction of short axis. The rubbing directions 305 and 306 meet at right angles with each other. The liquid crystals are levo-rotary twist oriented. The distance (s) is 2.0 xcexcm between the first pixel electrode 303a and the second pixel electrode 303b. The pitch among the pixels is 18 xcexcm. FIG. 3 shows the results of simulation. The first pixel electrode, second pixel electrode and opposing electrode are provided on a light-transmitting substrate.
Further, the structure without dielectric on the ends of the first pixel electrode and the second pixel electrode was simulated by using the simulation model of FIG. 19. The simulating conditions were the same as those of the simulation model of FIG. 2 except that no dielectric was used. The same elements as those of FIG. 2 are denoted by the same reference numerals. The simulated results are shown in FIG. 20.
According to the simulated results of FIG. 20, the orientation of liquid crystals is shown by a two-dimensional cross section. There are shown equipotential lines, liquid crystal director and transmission factor. The transmission factor indicates the leakage of light from the end of the first pixel electrode in a width of 3.4 xcexcm. It is further learned that there is a disclination of a width of 3.6 xcexcm from the end of the second pixel electrode. The distance between the first pixel electrode and the second pixel electrode is 2.0 xcexcm and, hence, the sum (x) of width of the leakage of light and the disclination is 9.0 xcexcm.
According to the simulated results of FIG. 3 by providing the dielectric at the ends of the pixel electrodes, however, the equipotential lines are suppressed from being bent toward the pixel electrodes due to the dielectric of the high dielectric constant and, hence, the equipotential line components increase in parallel with the surfaces of the pixel electrodes. The sum (x) of widths of the disclination and the leakage of light was 7.5 xcexcm. The region where the disclination and the leakage of light have occurred decreased by 16% as compared to FIG. 20.
When the simulated results of FIG. 3 are compared with the simulated results of FIG. 20, it is learned that the sum (x) of widths of the disclination and the leakage of light is decreased by 1.5 xcexcm due to the formation of the dielectric at the ends of the pixel electrodes, the dielectric having a dielectric constant larger than a dielectric constant of liquid crystals in the direction of long axis. Since the pitch among the pixels is 18 xcexcm, the region where the disclination and the leakage of light occur is decreased by about 8% of the width of the pixel, and the numerical aperture can be improved.
The device was simulated in the simulation model of FIG. 2 by changing the height (h) of the dielectric under the following five conditions. The cell gap (d), the width (L) over which the dielectric is overlapped on the first pixel electrode and the width (L) over which the dielectric is overlapped on the second pixel electrode, vary depending upon the conditions. The dielectric possessed a relative dielectric constant of 30.
Condition (1): d=4.5 xcexc, L=1.0 xcexcm
Condition (2): d=4.5 xcexcm, L=2.0 xcexcm
Condition (3): d=3.0 xcexcm, L=1.0 xcexcm
Condition (4): d=2.0 xcexcm, L=0.2 xcexcm
FIG. 4 shows a relationship between the height of the dielectric and the sum of widths of the disclination and the leakage of light, wherein the abscissa represents a ratio of the height of the dielectric to the cell gap, and the ordinate represents the sum of the widths of the leakage of light and the disclination.
The condition (1) is compared below with the condition (2). That is, under the condition (1), the dielectric occupies a small proportion of the pixel electrode, and a small effect is exhibited for decreasing the disclination and the leakage of light. Under the condition (2), the dielectric is formed so as to be overlapped on the pixel electrodes over a width 1.3 xcexcm to 1.4 xcexcm close to the end thereof from a position at where the disclination and the leakage of light would occur when there is no dielectric. The disclination and the leakage of light are decreased by a width of a maximum of 1.5 xcexcm.
In driving the liquid crystal display device by applying a voltage thereto, a region where a black level of good quality is accomplished is the one where the equipotential lines are nearly in parallel with the surface of the pixel electrode. When the dielectric is provided on such a region, the leakage of light and the disclination rather increase due to the bending of the equipotential lines that stem from the contact of the dielectric having a different dielectric constant. Under the condition (4), therefore, the dielectric is provided slightly (by 0.5 xcexcm) on the inside of a position where the disclination and the leakage of light would occur when there is no dielectric. As compared to when there is no dielectric, therefore, the disclination and the leakage of light are decreased by a maximum of 0.5 xcexcm.
When the conditions (2), (3) and (4) are compared with one another, it is learned that the disclination and the leakage of light are markedly decreased by providing a dielectric of a high dielectric constant for those liquid crystal display devices having larger cell gaps. It is further learned that when the height of the dielectric is too large, the equipotential lines are excessively swollen toward the opposing electrode, and the disclination and the leakage of light rather increase.
The invention (1) is concerned with a liquid crystal display device comprising pixel electrodes, a dielectric overlapped on the ends of the pixel electrodes, an oriented film covering the pixel electrodes and the dielectric, and liquid crystals on the oriented film, the liquid crystals having a positive dielectric anisotropy, and the dielectric having a relative dielectric constant larger than a relative dielectric constant of the liquid crystals in the direction of long axis.
In the invention (2), the liquid crystals have a negative dielectric anisotropy, and the dielectric has a relative dielectric constant larger than the relative dielectric constant of the liquid crystals in the direction of short axis.
In both the invention (1) and the invention (2), the voltage is applied in a divided manner to an insulator of liquid crystals having a low dielectric constant in a circuit in which the insulator of liquid crystals of the low dielectric constant and a dielectric of a high dielectric constant are connected in series and are held between the pixel electrodes and the opposing electrode. By providing the dielectric of the high dielectric constant at the ends of the pixel electrodes, therefore, the equipotential lines are lifted up toward the opposing electrode. This suppresses the occurrence of the leakage of light and the disclination caused by the bending of equipotential lines toward the pixel electrodes at the ends of the pixel electrodes. To obtain this action, the relative dielectric constant of the dielectric provided at the ends of the pixel electrodes must be larger than the relative dielectric constant of the liquid crystals.
The invention (3) is concerned with a liquid crystal display device comprising pixel electrodes, a dielectric overlapped on the ends of the pixel electrodes, an oriented film covering the dielectric and the pixel electrodes, and liquid crystals on the oriented film, the dielectric having a relative dielectric constant of not smaller than 20.
In the invention (3), it is desired that the dielectric has a relative dielectric constant which is not smaller than 20, so that the relative dielectric constant of the dielectric is larger than the relative dielectric constant of the liquid crystals as considered from a general dielectric constant of the liquid crystals.
In the case of the nematic liquid crystals having a positive dielectric anisotropy, the relative dielectric constant of the liquid crystals in the direction of long axis is usually from about 8 to about 20. In the case of the liquid crystal display device using nematic liquid crystals having the positive dielectric anisotropy, therefore, it is considered that the relative dielectric constant of the dielectric needs be not smaller than 20.
In the case of the nematic liquid crystals having a negative dielectric anisotropy, the relative dielectric constant of the liquid crystals in the direction of short axis is usually from about 8 to about 20. In the case of the liquid crystal display device using nematic liquid crystals having the negative dielectric anisotropy, therefore, it is considered that the relative dielectric constant of the dielectric needs be not smaller than 20.
The invention (4) is concerned with a liquid crystal display device comprising pixel electrodes, a dielectric overlapped on the ends of the pixel electrodes, an oriented film covering the dielectric and the pixel electrodes, and liquid crystals on the oriented film, the dielectric having a relative dielectric constant of not smaller than 30.
In the invention (4), the relative dielectric constant of the dielectric is selected to be 30 to observe the effect of greatly decreasing the disclination and the leakage of light in the simulation by using the model of FIG. 2. The higher the dielectric constant of the dielectric, the larger the effect for lifting up the, toward the opposing electrode, the equipotential lines that bend toward the pixel electrodes at the ends of the pixel electrodes. Therefore, the effect for greatly decreasing the disclination and the leakage of light is obtained even when the dielectric has a relative dielectric constant which is larger than 30.
The inventions (5) to (8) comprise pixel electrodes, an oriented film on the pixel electrodes, a dielectric on the ends of the pixel electrodes and liquid crystals on the oriented film and on the dielectric, making a difference from the inventions (1) to (4). Even by forming the oriented film which is an insulator on the pixel electrodes and by forming the dielectric thereon, the equipotential lines can be lifted by the dielectric at the ends of the pixel electrodes toward the opposing electrodes. In the liquid crystal display device using liquid crystals having a positive dielectric anisotropy, the relative dielectric constant of the dielectric must be larger than the relative dielectric constant of the liquid crystals in the direction of long axis, as a matter of course. In the liquid crystal display device using liquid crystals having a negative dielectric anisotropy, the relative dielectric constant of the dielectric must be larger than the relative dielectric constant of the liquid crystals in the direction of short axis. The relative dielectric constant of the dielectric may be selected to be not smaller than 20 considering from a general dielectric constant of the liquid crystals. As the effect is confirmed by simulation, the relative dielectric constant of the dielectric may be selected to be not smaller than 30.
The invention (9) is concerned with the liquid crystal display device of (4) or (8), wherein the cell gap is not smaller than 2.0 xcexcm but is not larger than 4.5 xcexcm, and the height of the dielectric is not larger than 17% of the cell gap.
The invention (9) will now be described with reference to a graph of FIG. 4. The leakage of light and the disclination decrease with an increase in the height of the dielectric, become constant at a certain height of the dielectric and, then, rather increase as the height of the dielectric further increases. In the liquid crystal display device having the cell gap which is not smaller than 2.0 xcexcm but is not larger than 4.5 xcexcm, the disclination and the leakage of light rather increase as the dielectric becomes too high. When the height of the dielectric is not larger than 17% of the cell gap, however, the leakage of light and the disclination decrease monotonously with an increase in the height of the dielectric.
The invention (10) is concerned with a liquid crystal display device of any one of (1) to (5), comprising an opposing electrode provided facing the pixel electrodes, and an oriented film formed on the opposing electrode, wherein a gap is maintained between the dielectric and the oriented film formed on the opposing electrode.
In this invention, the dielectric provided at the ends of the pixel electrodes is different from a spacer that is provided for maintaining the cell gap of the liquid crystal display device to be of a predetermined thickness.
The invention (11) is concerned with a liquid crystal display device of any one of (1) to (8), wherein the dielectric is an oxide containing titanium or tantalum. For example, a ditantalum pentoxide (Ta2O5) and a titanium dioxide (TiO2) have relative dielectric constants of as high as 30 or larger, and can be used as the dielectric of the invention.
The thus determined structure of the pixel portion of the invention is for bringing the lines of electric force of when an electric field is applied to be perpendicular to the flat surface on where the pixel electrodes are formed, and can be widely used as means for decreasing the defective orientation of liquid crystals in both the orientation system of the normally white mode and the orientation system of the normally black mode.
If defective orientation of liquid crystals due to ruggedness is not induced, this invention can be applied to the orientation system that uses smectic liquid crystals. For example, the invention can be applied to the liquid crystal display devices using ferroelectric liquid crystals and anti-ferroelectric liquid crystals. The invention can be further applied to a liquid crystal display device using a material cured by adding liquid crystalline high molecules to the smectic liquid crystals followed by the irradiation with light (e.g., ultraviolet rays).
The constitution of the pixel portion of the invention can be widely used as means for adjusting the electric field distribution in the display device which optically modulates the dimmer layer by applying a voltage to the dimmer layer through the semiconductor elements.
In the liquid crystal display device of the projection type, in particular, the leakage of light and the disclination are projected onto the screen being enlarged through an optical system that uses lenses. Therefore, this invention is particularly effective in the liquid crystal display device of the projection type.
The effect of the invention can be exhibited to a sufficient degree even when there is formed an inorganic film having a function for preventing the short-circuiting as an insulating film between the upper surfaces of the pixel electrodes and the oriented film. Presence of the dielectric on the ends of the pixel electrodes still makes it possible to prevent the equipotential lines from bending toward the pixel electrodes.